Electrostatic coating apparatus



June 5, 1962 F. A. CROSKEY ETAL 3,037,7

ELECTROSTATIC COATING APPARATUS 2 Sheets-Sheet 1 Filed Apr il 6. 1960 I INVENTORS jmm. 00.91%;(5 BY paa/ cf/gkmmv ATTORNEY June 5, 1962 F. A. CRQSKEY ETAL 3,037,703

' ELECTROSTATIC COATING APPARATUS Filed April 6, 1960 2 Sheets-Sheet 2 IN VEN T 0R5 ATTORNEY United States 3,037,703 ELECTROSTATIC COATENG APPARATUS Frank A. Croskey, New Baltimore, and Paul J. Rissman,

Dearborn, Mich., assignors to General Motors Corporation, Detroit, Mich., a corporation of Delaware Filed Apr. 6, 1960, Ser. No. 20,453 13 Claims. (Cl. 239) This invention relates to electrostatic coating apparatus, and more particularly to improved electrostatic spray painting apparatus.

One feature of the invention is that it provides improved electrostatic spray painting apparatus; another feature of the invention is that it provides spray emitting or atomizing means, as a spray gun, having improved means for charging liquid coating material with a high voltage before it is atomized. A further feature of the invention is that it provides apparatus wherein liquid coating material flows through inlet and outlet chambers which are connected by a long restricted passage wherein the liquid coating material is charged with a high voltage. Still another feature of the invention is that the restricted passage may be in the form of a spiral having a length substantially greater than the length of the inlet and outlet chambers. A further feature of the invention is that means are provided to adjust the dielectric length of the restricted passage, and still another feature of the invention is that the liquid coating material is sprayed from a charged conductive nozzle which is substantially entirely surrounded by a dielectric shield to confine the ionizing area to the location of the nozzle.

Other features and advantages of the invention will be apparent from the following specification and drawings, in which:

FIGURE 1 is a side elevational view partly in vertical section through apparatus embodying our invention; and

FIGURE 2 is an enlarged view partly in vertical section through a portion of the apparatus of FIGURE 1.

In the copending application of Frank A. Croskey, entitled Electrostatic Spray Painting Apparatus, filed September 21, 1959, as Serial No. 841,140, there is disclosed an arrangement wherein spray emitting means, as a spray gun, is encased in a dielectric housing and wherein the liquid coating material is fed to the spray emitting means through a passage which forms part of a high voltage electric circuit so that the coating material becomes charged to a high voltage before it reaches the spray nozzle.

The present invention comprises refinements and improvements upon the apparatus of SN. 841,140. According to the present invention, liquid coating material flows toward the nozzle through a passage including an inlet chamber and an outlet chamber connected by a long restricted passage portion. The outlet chamber and the nozzle are connected to the high voltage source and the inlet chamber is connected to ground so that the liquid coating material is charged with a high voltage as it flows through the restricted passage portion. Preferably, the restricted passage portion is formed as a spiral and there is a spiral conductor mounted in the inlet chamber and adjustably threaded into the restricted passage portion to vary the effective dielectric length thereof. The conductor is of smaller cross-sectional area than is the restricted passage portion to permit flow of liquid coating material through the restricted portion past the conductor, and control means are provided for turning the conductor into and out of the restricted portion to vary its eifective dielectric length. Coating material flows from the outlet chamber through a passage including a conductor having a bore defined by a sharp annulus located in the outlet chamber to concentrate the field, and the nozzle (which 3,037,703 Patented June 5, 1962 is conductive) is formed with only a small exposed area which is Surrounded by a dielectric shield to confine the ionizing area to the conductive nozzle alone.

Referring now more particularly to the drawings, a spray emitting means which comprises a spray gun generally designated as 2 is mounted inside a hollow molded epoxy resin casing 4 having a back cover 6 which is secured to the casing walls by a plurality of bolts, one of which is shown at 8.

The spray gun 2 illustrated herein is a conventional spray gun manufactured by Binks Manufacturing Company and designated by the manufacturer as Model No. 21. While the gun has been modified in a manner to be described, the general construction and operation of the spray gun are conventional and are well known in the art. A gun of this type is shown and described in detail in Harrison et al. Patent 2,266,365, to which reference may be had for a more detailed explanation of the conventional structure and operation.

The gun has a metallic body 10 to which is fed liquid coating material to be atomized, low pressure atomizing air and high pressure air for operating a needle valve 12. The system illustrated is an automatic apparatus which is adapted to be mounted on a stand 14 by means of a set screw 16 threaded into a boss 18 which projects from the casing 4.

Atomizing air is fed to the gun through a nylon tube 20 which extends from the gun out through the cover 6 of the casing, and high pressure air for operating the needle valve is fed to the gun through a nylon tube 22 which projects from the gun out through the cover 6. The use of nylon tubes for the air feed prevents electron loss through these feed devices as explained in more detail in application S.N. 841,140. When the high pressure air is supplied to the gun, the needle valve 12 is retracted. Liquid coating material flowing through the nozzle 34 is atomized by low pressure atomizing air emitted from orifices in an air nozzle 36.

The metallic gun 2 is charged to a high voltage, preferably in the order of 35 kv. to 70 kv., from a DC. power source 24 which is connected to the body of the gun through a coaxial cable 26 which plugs into the jack of a connector 23 housed in a nylon high voltage cable retainer 30 which projects out through the cover 6 of the casing. Preferably, the gun is charged negatively.

The nozzle 34 of the gun 2 projects out of the casing 4 through an opening 4a within which is mounted a neoprene grommet 32 which serves to prevent corona leakage from the metal nozzle and which also prevents cleaning solvents from entering the casing 4 through the opening 4a. The nozzle 34 is modified from its conventional form by forming a sharp edge around the liquid discharge orifice to provide the smallest possible metallic ionization area. The nozzle is surrounded by a dielectric air nozzle or shield 36. It is hi hly undesirable to use a metallic air nozzle in this system and preferably the air nozzle 36 is formed of Micarta or nylon. The air nozzle completely surrounds the metallic liquid nozzle 34 except for the liquid emitting opening itself so that the ionization area is confined to the area of liquid discharge from the metallic nozzle 36.

Paint or other liquid coating material is fed by a pipe 37 leading from a supply source through a passage which is in the electric circuit and wherein the liquid coating material is charged with a high voltage. The passage is formed in an elongated cylindrical dielectric housing member 33 which preferably is formed of nylon and which receives the supply pipe 37 through a liquid coating material inlet opening 40 in its cylindrical wall adjacent one end thereof. The inlet opening communicates with an elongated inlet chamber 42 formed as a bore in the housing member 38. An enlarged counterbore 44 com municates with the downstream end of the inlet chamber 42 and a dielectric insert 46 is mounted in the counterbore. The insert 46, which preferably is formed of a plastic or ceramic material, has a restricted spiral passage 48 extending through its entire length and opening into each opposite end of the insert so that one end of the restricted spiral passage communicates with the inlet chamber 42 while the other end communicates with the base of a conical outlet chamber 50 formed in a nylon insert 52 which is mounted in the end of the counterbore 44. As may be seen from FIGURE 2, the restricted spiral passageway 48 is substantially longer than either of the chambers 42 or 50 with which it communicates and the cross-sectional area of the spiral passageway is much smaller than the cross-sectional area of either of the chambers 42 or 50 so that the spiral passageway provides a restriction in the passage through which the coating material flows.

A conductive cap 54 projects into the conical chamber 50 of the nylon insert at the apex of the chamber 50. The cap 54 has a central bore 54a communicating with the conical chamber 50 and forming par-t of the passageway for the coating material and, as best shown in FIGURE 2, the end of the cap 54 which is within the conical chamber 50 is formed as a sharp annulus to concentrate the high voltage field and facilitate electron flow to insure maximum charging of the liquid coating material. In order to avoid the eifects of sedimentation, the cap is formed with a plurality of bypass openings 54b which extend between the conical chamber 50 and the bore 54a of the cap as shown in FIG. 2.

The downstream end of the cap 54 seats in a well in the end of a conductive member 56 which preferably is formed of brass and which has a bore 56a forming part of the liquid coating material passageway. At the downstream end of the brass casting 56 the paint passageway extends through the bore of a brass paint coupler 58 which is seated in a recess in the body of the spray gun 2, and which communicates with a passage 60 leading to the nozzle 34. The needle valve 12 is mounted in the terminal portion of the pass-age 60 and serves to seal the end of the passage against flow of coating material when the high pressure air is off.

A spiral conductor 62, which may be formed of spring steel wire, is mounted in the inlet chamber 42 and is adjustably threaded into the restricted spiral passage 48 to vary the effective dielectric length thereof. As shown in FIGURE 1, the conductor 62 is of smaller cross-sectional area of the spiral passage to permit flow of liquid coating material through the spiral passage past the conductor. Control means are connected to the conductor for turning it into and out of the restricted portion of the passage to vary the effective dielectric length thereof. As shown in FIGURE 1, there is a metal control rod 64 which is journaled through a sealing member 66 in the end of the nylon housing 38. On the outer end of the rod 64 is mounted a knurled knob 68 so that the rod can be turned by hand.

The portion 64a of the rod which projects inside the inlet chamber 42 is flattened on one side so that a spring seat member 70 may slide longitudinally on the rod but is fixed against rotation relative thereto, the spring seat member having an opening complementary to and receiving the rod portion 64a with the flattened side. The spring seat member 70 is secured to the upstream end of the spiral conductor 62. When the knob 68 is turned, the conductor 62 rotates with the rod and is moved into or out of the spiral passage 48 to vary the effective dielectric length of the passage. The spiral conductor also provides a means for cleaning the spiral passage 48.

An electrical brush 72 is mounted in the sealing member 66 and makes sliding contact with the metal control rod 64. A conductor 74 connects the brush 72 to ground. Since the entire body of the gun 2 is charged to a high potential from the power source 24, the conductive cap 54 is also charged since it is connected through the brass casting 56 to the body of the gun. The annular terminus of the cap 54 which is inside the conical chamber 50 is formed as a sharp edge to concentrate the electrical charge, and coating material flowing through the inlet chamber 42, the spiral passage 48 and the conical chamber 5!), is charged to a high potential.

The amount of current flowing through the coating material may be controlled by adjustment of the spiral conductor 62. We have found that a coating material that will conduct a steady current of at least 40 microamperes may be charged satisfactorily with this apparatus. Coating material that will conduct a steady current of microamperes or more will be charged to provide a greater efficiency of operation, providing that the spiral conductor 62 is backed out of the spiral passageway 48 until the current flow is reduced to about two microamperes before atomization is set into eifect. Once atomization is started, the current increases due to the current flow through the spray cloud to the object being coated.

This apparatus may be used effectively to coat articles with certain paint materials which heretofore have proved to be unsuitable for use in an electrostatic system. Such paint materials as metallics and water soluble paints which have been found unsuitable for use in other electorstatic coating systems, can be charged sufliciently in the apparatus herein described to provide efiicient coating.

While we have shown and described one embodiment of our invention, it is capable of many modifications. Changes, therefore, in the construction and arrangement may be made without departing from the spirit and scope of the invention, as set forth in the appended'claims.

We claim:

1. Electrostatic coating apparatus of the character described, including: spray emitting means having a nozzle; means for charging said spray emitting means with a high voltage; means for supplying liquid coating material to said spray emitting means; means providing a dielectric passage through which said liquid passes toward said nozzle, said passage including a restricted portion of spiral configuration; a spiral conductor adjustably threaded into the restricted portion of said passage, said conductor being of smaller cross-sectional area than the cross-sectional area of the restricted portion of the passage to permit flow of liquid coating material through said restricted portion past said conductor; control means connected to said conductor for turning the conductor into and out of the restricted portion of said passage to vary the eiiective dielectric length thereof; means connecting the downstream end of said passage to said charging means; and means connecting the upstream end of said passage to a source of electrical potential which differs from the potential of said charging means, whereby current flows through the liquid coating material in said passage to charge the liquid with a high voltage.

2. Apparatus of the character claimed in claim 1, wherein the upstream end of said passage is connected to ground.

3. Electrostatic coating apparatus of the character described, including: spray emitting means having an electrically conductive nozzle; means for charging said spray emitting means and nozzle with a high voltage; means for supplying liquid coating material to said spray emitting means; means providing a dielectric passage through which said liquid passes toward said nozzle, said passage including a restricted portion of spiral configuration; a spiral conductor adjustably threaded into the restricted portion of said passage, said conductor being of smaller crosssectional area than the cross-sectional area of the restricted portion of the passage to permit fiow of liquid coating material through said restricted portion past said conductor; control means connected to said conductor for turning the conductor into and out of the restricted portion of said passage to vary the efiective dielectric length thereof; means connecting the downstream end of said passage to said charging means; and means connecting the upstream end of said passage to ground, whereby current flows through the liquid coating material in said passage to charge the liquid with a high voltage.

4. Electrostatic coating apparatus of the character described, including: spray emitting means having an elec trically conductive nozzle with an orifice for emitting liquid, said spray emitting means including a dielectric shield which contacts the surface of the body of the nozzle and substantially entirely encompasses the nozzle except for the liquid emitting orifice of the nozzle; means for charging said spray emitting means and nozzle with a high voltage; means for supplying liquid coating material to said spray emitting means; means providing a dielectric passage through which said liquid passes toward said nozzle, said passage including a restricted portion of spiral configuration; means connecting the downstream end of said passage to said charging means; and means connecting the upstream end of said passage to a source of electrical potential which differs from the potential of said charging means, whereby current flows through the liquid coating material in said passage to charge the liquid with a high voltage.

5. Electrostatic coating apparatus of the character described, including: spray emitting means having an electrically conductive nozzle with an orifice for emitting liquid, said spray emitting means including a dielectric shield which substantially entirely surrounds the nozzle except for the liquid emitting orifice; means for charging said spray emitting means with a high voltage; means for supplying liquid coating material to said spray emitting means; means providing a dielectric passage through which said liquid passes toward said nozzle, said pasage including a restricted portion of spiral configuration; a spiral conductor adjustably threaded into the restricted portion of said passage, said conductor being of smaller cross-sectional area than the cross-sectional area of the restricted portion of the passage to permit flow of liquid coating material through said restricted portion past said conductor; control means connected to said conductor for turning the conductor into and out of the restricted portion of said passage to vary the eifective dielectric length thereof; means connecting the downstream end of said passage to said charging means; and means connecting the upstream end of said passage to ground whereby current flows through the liquid coating material in said passage to charge the liquid with a high voltage.

6. Electrostatic coating apparatus of the character described, including: spray emitting means having a nozzle; means for charging said spray emitting means with a high voltage; means for supplying liquid coating material to said spray emitting means; means providing a dielectric passage through which liquid passes toward said nozzle, said passage including a first chamber adjacent one end thereof, a restricted passage portion of spiral configuration, one end of which communicates with said first chamber, and a second chamber in communication with the other end of said spiral passage portion, the spiral portion being of smaller cro-sssectional area than either of said chambers; means connecting the downstream end of said passage to said charging means; and means connecting the upstream end of said passage to a source of electric potential which differs from the potential of said charging means, whereby current flows through the liquid coating material in said passage to charge the liquid with a high voltage.

7. Electrostatic coating apparatus of the character described, including: spray emitting means having an electrically conductive nozzle; means for charging said spray emitting means and nozzle with a high voltage; means for supplying liquid coating material to said spray emitting means; means providing a dielectric passage through which said liquid passes toward said nozzle, including an elongated dielectric member having a chamber near one end thereof, a dielectric insert mounted in the dielectric member and having a spiral :bore of smaller cross-sectional area than said chamber, one end of said bore communicating with said chamber, a second dielectric insert mounted in the dielectric member and having an internal chamber, the base of which is of substantially greater cross-sectional area than said spiral bore and communicates with the other end of said spiral bore; means connecting the downstream end of said passage to said charging means; and means connecting the upstream end of said passage to a source of electrical potential which differs from the potential of said charging means, whereby current flows through the liquid coating material in said passage to charge the liquid with a high voltage.

8. Electrostatic coating apparatus of the character described, including: spray emitting means having an electrically conductive nozzle; means for charging said spray emitting means and nozzle with a high voltage; means for supplying liquid coating material to said spray emitting means; means providing a dielectric passage through which said liquid passes toward said nozzle, including an elongated dielectric member having a chamber near one end thereof, a dielectric insert mounted in the dielectric member and having a spiral bore of smaller cross-sectional area than said chamber, one end of said bore communicating with said chamber, a second dielectric insert mounted in the dielectric member and having an internal conical chamber, the base of which is of substantially greater cross-sectional area than said spiral bore and communicates with the other end of said spiral bore; a spiral conductor having a portion adjustably threaded into said spiral bore, said conductor being of smaller cross-sectional area than the cross-sectional area of the spiral bore to permit flow of liquid coating material through said bore past said conductor; control means connected to said conductor for turning the conductor into and out of the spiral bore to vary the eifective dielectric length thereof; means connecting the downstream end of said passage to said charging means; and means connecting the upstream end of said passage to ground, whereby current flows through the liquid coating material in said passage to charge the liquid with a high voltage.

9. Electrostatic coating apparatus of the character described, including: spray emitting means having a nozzle; means for charging said spray emitting means and nozzle with a high voltage; means for supplying liquid coating material to said spray emitting means; means providing a dielectric passage through which said liquid passes toward said nozzle, including an elongated dielectric member having a chamber near one end thereof, a dielectric insert mounted in the dielectric member and having a spiral bore of smaller cross-sectional area than said chamber, one end of the bore communicating with the chamber, a second dielectric insert mounted in the dielectric member and having an internal conical chamber, the base of which is of substantially greater cross-sectional area than the crosssectional area of said spiral bore and which communicates with the other end of said spiral bore, a conductive cap mounted on said second insert and projecting into said conical chamber at the apex thereof, said cap having a bore terminating in a sharp annular end in said conical chamber, and said cap being formed with a plurality of bypass openings extending between the conical chamber and the bore of the cap; means connecting the downstream end of said passage to said charging means; and means connecting the upstream end of said passage to a source of electrical potential which difiers from the potential of said charging means, whereby current flows through the liquid coating material in said passage to charge the liquid with a high voltage.

10. Electrostatic coating apparatus of the character described, including: spray emitting means having a nozzle; means for charging said spray emitting means and nozzle with a high voltage; means for supplying liquid coating material to said spray emitting mean-s; means providing a dielectric passage through which said liquid passes toward said nozzle, including an elongated dielectric memher having a chamber near one end thereof, a dielectric insert mounted in the dielectric member and having a spiral bore of smaller cross-sectional area than said chamber, one end of the bore communicating with the cham her, a spiral conductor in said chamber having a portion adjustably threaded into said spiral bore, the conductor being of smaller cross-sectional area than the cross-sectional area of the spiral bore to permit flow of liquid coating material through said spiral bore past said conductor, control means connected to said conductor for turning the conductor into and out of the spiral bore to vary the effective dielectric length thereof; means connecting the downstream end of said passage to said charging means; and means connecting the upstream end of said passage to ground, whereby current flows through the liquid coating material in said passage to charge the liquid with a high voltage.

11. Electrostatic coating apparatus of the character described, including: spray emitting means having a conductive nozzle with a small exposed area; means for charging said spray emitting means with a high voltage; means for supplying liquid coating material to said spray emitting means; means providing a dielectric passage through which said liquid passes toward said nozzle, said passage including a first chamber connected to the liquid coating material inlet, a second chamber connected to the nozzle, and a long restricted passage having one end opening into said first chamber and the other end opening into said second chamber; means connecting said second chamber and said nozzle to said charging means; and means connecting said first chamber to a source of electrical potential which differs from the potential of said charging means, whereby current flowsthrough the liquid coating Q 1., material in said passage to charge the liquid with a high voltage.

12. Electrostatic coating apparatus of the character described, including: spray emitting means having a conductive nozzle with a small exposed area; means for charging said spray emitting means with a high voltage; means for supplying liquid coating material to said spray emitting means; means providing a dielectric passage through which said liquid passes toward said nozzle, said passage including a first chamber connected to the liquid coating material. inlet, a second chamber connected to the nozble, and a long restricted passage having one end opening into said first chamber and the other end opening into said second chamber; a conductor adjustably mounted in said restricted passage; said conductor being of smaller cross-sectional area than the cross-sectional area of the restricted passage to permit flow of liquid coating material through said restricted passage past said conductor, control means connected to said conductor for moving it into and out of the restricted passage to vary the elfective dielectric length thereof; means connecting said second chamber and said nozzle to said charging means; and means connecting said first chamber to ground, whereby current flows through the liquid coating material in said passage to charge the liquid with a high voltage.

13. Apparatus of the character claimed in claim 12, Where in said restricted passage is formed as a spiral having a length substantially greater than the length of either of said chambers.

References Cited in the file of this patent UNITED STATES PATENTS Littlefield Apr. 19, 1932 Sedlacsik Mar. 5, 1957 Sedlacsik Nov. 17, 1959 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,037,703 June 5, 1962 Frank A. Croskey et al.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 3, line 50, after "area" insert thanthe cross-sectional area Signed and sealed this 16th day of October 1962.

(SEAL) Attest:

ERNEST w. swmsa A L- LADD Attesting Officer Commissioner of Patents 

